I used to have a 2011 Cyclone 370C. Not the most insulated camper but it did "fair". We now have a 2015 Vengeance. Its amazing how much better insulated it is. Maybe its the manufacturer but for the most part, I think its just using better materials these days.
GDs are going to be just as good or better than most. Yes, its going to be hot. They're certainly not a house. But they should be pretty well. If GD offers a particular package for extreme temps, obviously get it. But like ramgunner said, 3 ACs help a ton.
We also take at least one or two fans. Keeping the air moving around makes a nice difference.